Power supply circuit and electronic equipment

ABSTRACT

A power factor is improved using a simplified structure without particularly providing a power factor improving circuit, and high efficiency is obtained. For this purpose, one terminal of a direct-current power source  12 , from which a pulsating current is obtained, is connected through a choke coil  13  to one end of an auxiliary coil  14   c  constituting a converter transformer  14 ; the other end of the auxiliary coil  14   c  is connected to the other terminal of the direct-current power source  12  through a series circuit of a diode  15  and a capacitor  16 ; a connection midpoint between the diode  15  and the capacitor  16  is connected to one end of a primary coil  14   a  of the converter transformer  14 ; the other end of the primary coil  14   a  is connected to the other terminal of the direct-current power source  12  through a switching element  17 ; a secondary coil  14   b  of the converter transformer  14  is connected to direct-current voltage output terminals  19   a  and  19   b  through a rectifying circuit  18 ; the direct-current voltage output terminal  19   a  is connected to an input side of a pulse width modulation control circuit  20 ; and an output terminal of the pulse width modulation control circuit  20  is connected to a control electrode of the switching element  17.

TECHNICAL FIELD

The present invention relates to a power supply circuit capable ofsupplying comparatively large electric power and electronic equipmentwhich uses comparatively large electric power.

BACKGROUND ART

Conventionally, as a power supply circuit of a switching type capable ofsupplying comparatively large electric power of, for example, 70 W, oneshown in FIG. 7 has been proposed. The power supply circuit shown inFIG. 7 is the one to which a power factor improving circuit is added tocomply with a harmonics regulation.

In FIG. 7, a reference numeral 1 denotes, for example, a commercialpower supply of 100v, 50 Hz, and one terminal and the other terminal ofthe commercial power supply 1 are respectively connected to one terminaland the other terminal on the input side of a rectifying circuit 3having a bridge structure of diodes through a high frequency blockingfilter 2.

A pulsating current in the positive direction corresponding to afrequency of the commercial power supply 1 is obtained at a positiveelectrode terminal and a negative electrode terminal on the output sideof the rectifying circuit 3. The positive electrode terminal of theoutput side of the rectifying circuit 3 is connected to one end of aprimary coil 5 a of a converter transformer 5 through a series circuitof a choke coil 4 a and a diode 4 b constituting a power factorimproving circuit 4; the other end of the primary coil 5 a is connectedto a drain of a field effect transistor 6 constituting a switchingelement; and a source of the field effect transistor 6 is connected tothe negative electrode terminal on the output side of the rectifyingcircuit 3.

A connection midpoint between the choke coil 4 a and the diode 4 b isconnected to a drain of a field effect transistor 4 c constituting thepower factor improving circuit 4; a source of the field effecttransistor 4 c is connected to the negative electrode terminal of therectifying circuit 3; and a switching signal from a control circuit 4 dis supplied to a gate of the field effect transistor 4 c. In addition, aconnection midpoint between the diode 4 b and the one end of the primarycoil 5 a is connected to the negative electrode terminal of therectifying circuit 3 through a capacitor 4 e constituting the powerfactor improving circuit 4.

In the power factor improving circuit 4, the pulsating current obtainedat the output side of the rectifying circuit 3 is made into the form ofa sine wave to be supplied to the primary coil 5 a of the convertertransformer 5.

Further, one end of a secondary coil 5 b having a winding of a reversephase to the primary coil 5 a of the converter transformer 5 isconnected to a direct-current voltage output terminal 8 a of one endthrough a diode 7 a constituting a rectifying circuit 7; a connectionmidpoint between the diode 7 a and the direct-current voltage outputterminal 8 a of one end is connected to the other end of the secondarycoil 5 b through a smoothing capacitor 7 b constituting the rectifyingcircuit 7; and the other end of the secondary coil 5 b is connected tothe direct-current voltage output terminal 8 b of the other end.

The direct-current voltage output terminal 8 a of one end is connectedto the input side of a pulse width modulation control circuit 9consisting of a semiconductor integrated circuit; a switching signal ofa pulse width modulation signal obtained on the output side of the pulsewidth modulation control circuit 9 is supplied to a gate of the fieldeffect transistor 6; and switching of the field effect transistor 6 isperformed using the switching signal of the pulse width modulationsignal, so that a constant direct-current voltage V₀ can be obtained onboth the one side and the other side of the direct-current voltageoutput terminals 8 a and 8 b.

In such power supply circuit as shown in FIG. 7, an input pulsatingcurrent from the rectifying circuit 3 is controlled by the power factorimproving circuit 4 to be the form of a sine wave, thereby improving apower factor.

As to the power factor in the above case, when an input electric poweris |W|; an input electric current is |A|; and an input electric voltageis |V|, then the power factor cos″ is expressed:cos″=|W|/(|A|×|V|)

When the power factor improving circuit 4 is provided as shown in FIG.7, the power factor cos″ can be improved up to 0.8 to 0.99 and an inputcurrent waveform is approximated to an input voltage waveform.

DISCLOSURE OF THE INVENTION

However, when the above power factor improving circuit 4 is providedwith a conventional power supply circuit of a switching type, efficiencyof the power factor improving circuit 4 is integrated with efficiency ofthe power supply circuit and therefore, the efficiency of the powersupply circuit is lowered. For example, if the efficiency of aconventional power supply circuit of a switching type is 90% and theefficiency of the power factor improving circuit 4 is 90%, the wholeefficiency becomes 81%.

Moreover, since the power factor improving circuit 4 performs switchingof a large electric current by using the field effect transistor 4 c,there is a disadvantage of the power factor improving circuit 4 becominga source which generates noise.

Further, when the power factor improving circuit 4 is provided, acircuitry becomes complicated with the power factor improving circuit 4and a space for accommodation thereof must be required, which leads to adisadvantage of the power supply circuit being more expensive.

In view of the above, the present invention aims to improve a powerfactor using a simplified structure without particularly providing apower factor improving circuit and also to obtain high efficiency.

A power supply circuit according to the present invention is the one inwhich one terminal of a direct-current power source capable of obtaininga pulsating current is connected through a choke coil to one end of anauxiliary coil constituting a converter transformer; the other end ofthe auxiliary coil is connected to the other terminal of thedirect-current power source through a series circuit of a diode and acapacitor; a connection midpoint between the diode and the capacitor isconnected to one end of an primary coil of the converter transformer;the other end of the primary coil is connected to the other terminal ofthe direct-current power source through a switching element; a secondarycoil of the converter transformer is connected to a direct-currentvoltage output terminal through a rectifying circuit; the direct-currentvoltage output terminal is connected to the input side of a pulse widthmodulation control circuit; and an output terminal of the pulse widthmodulation control circuit is connected to an control electrode of theswitching element.

According to the present invention, an input pulsating current flowswhen the diode is positively biased by a difference between a voltage ofthe auxiliary coil of the converter transformer, and acounter-electromotive voltage of the choke coil and an input voltage;and a waveform thereof corresponds to an input voltage of the pulsatingcurrent and a conduction angle is enlarged to becomes a form of a sinewave, so that an improvement of a power factor can be madeautomatically.

Further, according to the present invention, since there is no structurewhich brings the efficiency down, it is possible to obtain a powersupply circuit of a switching type which has high efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a constitutional diagram showing an embodiment of a powersupply circuit according to the present invention;

FIG. 2 is a circuit diagram provided for explaining FIG. 1;

FIG. 3 is a circuit diagram provided for explaining FIG. 1;

FIG. 4 is a constitutional diagram showing another embodiment accordingto the present invention;

FIG. 5 is a graph provided for explaining the present invention;

FIG. 6 is a graph provided for explaining the present invention; and

FIG. 7 is a constitutional diagram showing an example of a conventionalpower supply circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of a power supply circuit according to thepresent invention is explained referring to accompanied drawings.

FIG. 1 shows a power supply circuit according to this embodiment, inwhich a reference numeral 10 denotes a commercial power supply of, forexample, 100v, 50 Hz; and one terminal and the other terminal of thecommercial power supply 10 are respectively connected through a highfrequency blocking filter 11 to one terminal and the other terminal onthe input side of a rectifying circuit 12 having a bridge structure ofdiodes.

A pulsating current in a positive direction corresponding to a frequencyof the commercial power supply 10 can be obtained at a positiveelectrode terminal and a negative electrode terminal on the output sideof the rectifying circuit 12. The positive electrode terminal on theoutput side of the rectifying circuit 12 is connected through a chokecoil 13 to one end of an auxiliary coil 14 c having a winding of areverse phase to a primary coil 14 a constituting a convertertransformer 14; the other end of the auxiliary coil 14 c is connected toan anode of a diode 15; a cathode of the diode 15 is connected to apositive electrode of an electrolytic capacitor 16; and a negativeelectrode of the electrolytic capacitor 16 is connected to the negativeelectrode terminal on the output side of the rectifying circuit 12.

A connection midpoint between the cathode of the diode 15 and thepositive electrode of the electrolytic capacitor 16 is connected to oneend of the primary coil 14 a of the converter transformer 14; the otherend of the primary coil 14 a is connected to a drain of a field effecttransistor 17 constituting a switching element; and a source of thefield effect transistor 17 is connected to the negative electrodeterminal of the rectifying circuit 12.

Further, one end of a secondary coil 14 b having a winding of a reversephase to the primary coil 14 a of the converter transformer 14 isconnected to an anode of a diode 18 a constituting a rectifying circuit18; a cathode of the diode 18 a is connected to a direct-current voltageoutput terminal 19 a of one end; a connection midpoint between the diode18 a and the direct-current voltage output terminal 19 a of one end isconnected to the other end of the secondary coil 14 b through asmoothing capacitor 18 b constituting the rectifying circuit 18; and theother end of the secondary coil 14 b is connected to a direct-currentvoltage output terminal 19 b of the other end.

The direct-current voltage output terminal 19 a of one end is connectedto the input side of a pulse width modulation control circuit 20consisting of a semiconductor integrated circuit; a switching signal ofa pulse width modulation signal corresponding to an outputdirect-current voltage V₀ obtained on the output side of the pulse widthmodulation control circuit 20 is supplied to a gate of the field effecttransistor 17; and switching is performed on the field effect transistor17 using the switching signal of the pulse width modulation signal, sothat the constant direct-current voltage V₀ is obtained at both the oneside and the other side of the direct-current voltage output terminals19 a and 19 b.

In the power supply circuit shown in FIG. 1, when the field effecttransistor 17 is ON, the circuit is as shown in FIG. 2, in which anelectric current i₁=V₁/L₁×t that is determined by an input pulsatingvoltage V₁ of the positive electrode terminal and the negative electrodeterminal of the rectifying circuit 12, and an inductance L1 of the chokecoil 13, tends to flow when the field effect transistor 17 is turned on;and the electric current i₁ flows for a period of time in which thediode 15 is positively biased, while reducing an electromotive voltageV₂ of the auxiliary coil 14 c of the converter transformer 14.

Therefore, in order to make the output direct-current voltage constant,the pulse width modulation control circuit 20 performs control so thatan electric current corresponding to the input pulsating voltage flowsin the field effect transistor 17 while fluctuating in the form of asine wave.

In other words, an amount of the reverse bias of the auxiliary coil 14 cby a magnetic flux of the converter transformer 14 is compensated. Thus,the input pulsating current can be detected and a fluctuation of theelectric current in the field effect transistor 17 changes an energystored in the choke coil 13.

Further, when the field effect transistor 17 is OFF, the circuit is asshown in FIG. 3, in which a counter-electromotive voltage V₃ of thechoke coil 13 is superimposed on the input pulsating voltage V₁ when thefield effect transistor 17 is turned off. Accordingly, an electricpotential at a connecting point A between the choke coil 13 and theauxiliary coil 14 c becomes a pulsating flow on which a switchingpotential corresponding to an amount of energy stored in the choke coil13 at a time of turn-on is superimposed.

On the other hand, a counter-electromotive voltage V₄, which isgenerated at a time of turn-off, is generated in the auxiliary coil 14 cof the converter transformer 14. The connecting point A has a reverseelectric potential to the electric potential of thecounter-electromotive voltage V₄, and an electric potential of aconnecting point B between the auxiliary coil 14 c and the diode 15becomes an electric potential of a difference thereof.

In other words: when V₁+V₃>V₄, an electric current i₃ flows through thediode 15; andwhen V ₁ +V ₃ <V ₄,the diode 15 is turned OFF and the electric current i₃ does not flow.This voltage V₃ is proportional to the electric current of the fieldeffect transistor 17 at the time of turn-on, when the pulsating currentis previously detected.

Since the power supply circuit of the present embodiment controls ON-OFFduty of the field effect transistor 17 and also the electromotivevoltage V₂ of the auxiliary coil 14 c at the time of ON and thecounter-electromotive voltage V₄ of the auxiliary coil 14 c at the timeof OFF in addition to the counter-electromotive voltage V₃ of the chokecoil 13 change in accordance with the input pulsating voltage V₁ and aload current i₄, it is easy to cope with the input pulsating voltage V₁and a load fluctuation by setting a voltage of the auxiliary coil 14 c.

As described above, the pulsating input current i₁ flows when the diode15 is positively biased by the difference between the voltage of theauxiliary coil 14 c of the converter transformer 14, and thecounter-electromotive voltage V₃ of the choke coil 13 and the inputpulsating voltage V₁; a waveform thereof becomes one which correspondsto the input voltage of the pulsating current; and a conduction angle isenlarged to become the form of a sine wave, so that a power factor canbe improved.

Therefore, according to the power supply circuit of this embodiment,there is an advantage of the power factor being improved automaticallyby performing the control to obtain a constant output direct-currentvoltage V₀.

Further, according to this embodiment, since a power factor improvingcircuit is not particularly provided in comparison to conventional powersupply circuits, efficiency is not lowered and a power supply circuithaving high efficiency can be obtained, so that power saving can berealized.

In this connection, examples of the power factor and efficiency areshown in FIGS. 5 and 6, where power supply having AC 100v, 50 Hz andhaving AC 240v, 50 Hz are respectively provided as the commercial powersupply 10 of this embodiment.

FIG. 5 shows a characteristic curve of the power factor a and acharacteristic curve of the efficiency b for an example in which AC100v, 50 Hz is provided as the commercial power supply 10. In this case,the power factor has been: 95.44% when an output current IO is 1 A;94.97% at 2 A; 93.89% at 4 A; and 92.66% at 5 A. Also, the efficiencyhas been: 88.97% when the output current IO is 1 A; 88.4% at 2 A; 87.3%at 4 A; and 86.8% at 5 A.

FIG. 6 shows a characteristic curve of the power factor c and acharacteristic curve of the efficiency d for an example in which AC240v, 50 Hz is provided as the commercial power supply 10. In this case,the power factor has been: 87.05% when the output current IO is 1 A;92.15% at 2 A; 92.21% at 4 A; and 93.12% at 5 A. Also, the efficiencyhas been: 83.26% when the output current IO is 1 A; 86.62% at 2 A;87.75% at 4 A; and 87.55% at 5 A.

FIG. 4 shows another embodiment according to the present invention. Theembodiment shown in FIG. 4 is an example in which the example of FIG. 1is further modified to attain higher efficiency. In order to explain theexample shown in FIG. 4, the same reference numerals are given to thoseportions corresponding to FIG. 1 and a redundant explanation will beomitted.

In the example of FIG. 4, the positive electrode terminal on the outputside of the rectifying circuit 12 is connected through the choke coil 13to one end of the auxiliary coil 14 c having a winding of a reversephase to the primary coil 14 a constituting the converter transformer14; the other end of the auxiliary coil 14 c is connected to the anodeof the diode 15; the cathode of the diode 15 is connected to thepositive electrode of the electrolytic capacitor 16 and the negativeelectrode of the electrolytic capacitor 16 is connected to the negativeelectrode terminal on the output side of the rectifying circuit 12.

A connection midpoint between the cathode of the diode 15 and thepositive electrode of the electrolytic capacitor 16 is connected to oneend of the primary coil 14 a of the converter transformer 14; the otherend of the primary coil 14 a is connected to the drain of the fieldeffect transistor 17 constituting the switching element; and the sourceof the field effect transistor 17 is connected to the negative electrodeterminal of the rectifying circuit 12.

In the example of FIG. 4, a connection midpoint between the other end ofthe primary coil 14 a and the drain of the field effect transistor 17 isconnected through a capacitor 21 to one end of a second auxiliary coil13 a having a winding of the same phase as the choke coil 13; the otherend of the second auxiliary coil 13 a is connected to a cathode of adiode 22; and an anode of the diode 22 is connected to the negativeelectrode terminal of the rectifying circuit 12.

Further, a connection point of the capacitor 21 and one end of thesecond auxiliary coil 13 a is connected to an anode of a diode 23; acathode of the diode 23 is connected to the connection midpoint betweenthe diode 15 and the electrolytic capacitor 16; and also the gate of thefield effect transistor 17 is connected to the negative electrodeterminal of the rectifying circuit 12 through a resistor 24. Otherstructure than the above remains the same as that of FIG. 1.

In such example of FIG. 4, the same operational effect as the example ofFIG. 1 is obtained and further, when the field effect transistor 17 isOFF, an electric current generated by a counter-electromotive power ofthe primary coil 14 a flows through the path of: the primary coil 14a→the capacitor 21→the diode 23→the electrolytic capacitor 16 to chargethe capacitor 21, and rising of a voltage at a portion where a voltageand an electric current of the field effect transistor 17 are crossed ismade slow, so that a switching loss of the field effect transistor 17can be reduced. When the field effect transistor 17 is ON, an electriccharge to the capacitor 21 flows through the path of: the capacitor21→the field effect transistor 17→the diode 22→the second auxiliary coil13 a→the capacitor 21; the capacitor 21 is discharged to return to aninitial state; and the switching loss at a time of next turn-off can bereduced.

Further, in this embodiment, such power supply circuits as that shown inFIGS. 1 and 4 are used for electronic equipment using comparativelylarge electric power of, for example, approximately 70 W or more. Inthis case, since this power supply circuit has high efficiency, it ispossible to reduce power consumption of the electronic equipment.

As described above, according to the present invention, the pulsatinginput current flows when a diode is positively biased by a differencebetween a voltage of an auxiliary coil of a converter transformer, and acounter-electromotive voltage of a choke coil and an input voltage; anda waveform thereof becomes one corresponding to the pulsating inputvoltage, a conduction angle is enlarged to be the form of a sine wave,and a power factor can be improved automatically.

In other words, according to a power supply circuit of the presentinvention, when performing control to obtain a constant outputdirect-current voltage, there is obtained an advantage of the powerfactor being improved automatically.

Further, according to the present invention, since a power factorimproving circuit is not particularly provided in comparison toconventional power supply circuits, a power supply circuit having highefficiency can be obtained without lowering the efficiency, so thatpower consumption can be reduced.

Moreover, in electronic equipment which uses the power supply circuitaccording to the present invention, since the power supply circuit hashigh efficiency, it is possible to reduce power consumption of theelectronic equipment.

It should be noted that though an example using 100v, 50 Hz is explainedas a commercial power supply in the above described example, it isobvious that other commercial power supply such as 90v to 264v, 50 Hzcan also be used.

Furthermore, the present invention is not limited to the above describedembodiments and needless to say various other structures could beadopted without departing from the spirit and scope of the presentinvention.

1. A power supply circuit comprising: a direct-current power source fromwhich a pulsating current is obtained; a choke coil connected to oneterminal of said direct-current power source; a converter transformerhaving a primary coil, a secondary coil and a first auxiliary coil; aswitching element which performs switching of an electric current ofsaid converter transformer; a pulse width modulation control circuitwhich performs on-off control of said switching element; a firstconnection means for connecting the other end of said choke coil to oneend of said first auxiliary coil of said converter transformer and forconnecting the other end of said first auxiliary coil to the otherterminal of said direct-current power source through a series circuit ofa first diode and a first capacitor; a second connection means forconnecting a connection midpoint between said first diode and firstcapacitor to one end of the primary coil of said converter transformerand for connecting the other end of said primary coil to the otherterminal of said direct-current power source through the switchingelement; a third connection means for connecting the secondary coil ofsaid converter transformer to a direct-current voltage output terminalthrough a rectifying circuit; and a fourth connection means forconnecting said direct-current voltage output terminal to the input sideof said pulse width modulation control circuit and for connecting anoutput terminal of the pulse width modulation control circuit to acontrol electrode of said switching element, wherein a connection pointof said primary coil and switching element is connected to theconnection point of said first diode and first capacitor through aseries circuit of a second capacitor and a second diode, and aconnection point of said second capacitor and second diode is connectedto a connection point of said switching element and the other terminalof said direct-current power source through a series circuit of a secondauxiliary coil having a winding of the same phase as said choke coil anda third diode.
 2. Electronic equipment using a comparatively largeelectric power, comprising the power supply circuit according to claim1.